1. Field of the Invention
The present invention relates to an ignition circuit for igniting a vehicle engine, and more particularly to a power transistor used in the circuit.
2. Description of the Related Art
FIG. 6 shows a conventional circuit. As an element for controlling a current flowing an ignition coil, a bipolar Darlington transistor 103 is used. As disclosed in Japanese Patent Examined Publication No. Sho 55-3538 and in U.S. Pat. No. 358,755, a current in a main circuit for current limitation is detected by connecting a resistor 106 to an emitter terminal of a bipolar Darlington transistor. As shown in FIG. 3 of U.S. Pat. No. 3,587,551, a transistor 104 for shunting a base current of the bipolar Darlington transistor 103 between the base of the Darlington transistor 103 and the ground of the resistor 106. The base terminal of the transistor 104 is connected to a point to which the main-circuit current detecting resistor 106 and the emitter terminal of the bipolar Darlington transistor 103 are connected.
A load current flowing an ignition coil 102 flows into the main-circuit-current detecting resistor 106 through the bipolar Darlington transistor 103. When a voltage drop in the main-circuit current resistor 106 generated by the current becomes about 0.6 V or more, the base-emitter voltage of the transistor 104 connected to the main-circuit current detecting resistor 106 also becomes about 0.6 V or more so that the transistor 104 operates to shunt a part of the base current of the bipolar Darlington transistor 104. As the base current of the bipolar Darlington transistor 103 decreases, the collector current which is a load current decreases. However, since the ignition coil 102 is a load having large inductance, the load current continues to flow, thereby enhancing the collector-emitter voltage of the bipolar Darlington transistor 103. As a result, the load current (i.e. collector current) becomes constant so that the voltage drop across the main-circuit current detecting resistor 106 is maintained constant (i.e. the operation of current limitation operates).
The resistor 111 and capacitor 112, which are not disclosed, serve to restrict the oscillation of current in the current limitation which is a known technique.
A driving circuit including resistors 107 and 108 and a transistor 105 is supplied with power from a battery 101, and when the transistor 105 is off, serves to cause the base current limited by the resistors 107 and 108 limited by the bipolar Darlington transistor 103 to flow into the bipolar Darlington transistor 103. But, the driving circuit should not be limited to the configuration of the driving circuit.
A Zener diode 110 is connected between the collector terminal and the base terminal of the bipolar Darlington transistor 103. The Zener diode 110 operates as follows. Because of the withstand voltage of the Zener diode 110 set at a lower voltage than that between the main terminals of the bipolar Darlington transistor 103, when the base current of the bipolar Darlington transistor 103 is removed so that it is turned off, an excess voltage applied from the ignition coil 102 passes a reverse current to the Zener diode 110. A part of the reverse current constitutes a base current of the bipolar Darlington transistor 103 so that the collector-emitter withstand voltage of the bipolar Darlington transistor 103 is substantially clamped by the Zener diode 110. This protects the bipolar Darlington transistor 103 from excess voltage. Then, almost all of the charges from the ignition coil is discharged as a collector current of the bipolar Darlington transistor 103. The zener diode 110 is illustrated in U.S. Pat. No. 4,030,469. An example of the method of fabricating the Zener diode 110 for the MOS gate structure transistor is disclosed in U.S. Pat. No. 5,115,369.
A configuration in which a capacitor is used in place of the Zener diode 110 is disclosed in Japanese Utility Model Examined publication No. Sho 55-48132, and used for protecting the transistor connected in series with the ignition coil.
FIG. 2 shows waveforms of a collector-emitter voltage and a corrector current before and after the current limitation operation by the bipolar Darlington transistor 103 in FIG. 6. As seen from the waveform shown in FIG. 2, at the position of the left side for paper, the collector-emitter voltage abruptly drops from 16 V to about 1 V. This timing is coincident with that when a base current not shown is supplied to the bipolar Darlington transistor 103. Thereafter, the collector current changes at the rate defined by a power supply voltage and the inductance of the ignition coil (the changing rate dic/dt per unit time=power supply voltage/inductance of ignition coil), but is subjected to the current limitation operation for limiting the collector current to a fixed value in the manner described in the prior art. The collector voltage while the collector current is limited is the result of subtraction of the voltage drop across the resistor element (mainly, resistance of the ignition coil) of the main circuit from the power supply voltage.
The explanation hitherto made applies to the case where the bipolar transistor is used as an element for controlling the ignition coil. In this case, where the transistor 105 and the resistor 108 in the driving circuit 109 of FIG. 6 are removed to realize the above function within a wide temperature range by using a 5 V series logic element, the large capacity 5 V series logic element having a current conducting capability of 20-50 mA is required although it depends on the current amplification factor of the bipolar Darlington transistor.
In order to miniaturize the entire ignition system, the driving current by the above 5 V series logic element is desired to be decreased by one order of magnitude. This can be easily accomplished by adopting a voltage-driving MOS gate structure transistor as an ignition coil current.
Where an existing MOS gate structure transistor (power MOSFET and IGBT) is applied to the circuit shown in FIG. 6, in the process in which the drain voltage abruptly increases at the time of initiation of current limitation as shown in FIG. 3, it becomes a value higher than the power supply voltage and also oscillates in an attenuation waveform. The optimization in the values of the resistor 111 and capacitor 112 and reduction of the rate (gain or amplification factor) of the output signal to the base signal of the transistor 104 in FIG. 6 is effective to suppress the current oscillation introduced when the collector current becomes constant, but is useless to prevent the above oscillation of the drain voltage.
FIG. 3 shows the waveforms of the voltage and current exhibiting the oscillation phenomenon. They are the waveforms of the drain voltage and drain current (ignition coil current) when the ignition coil current is controlled by the MOSFET with a withstand voltage of 250 V and driven by 5 V.
The oscillation of the drain voltage waveform as shown in FIG. 3 gives rise to the following problems.
(1) The voltage in proportion to the collector voltage oscillating toward the high voltage side of the ignition coil is induced so that sparking may occur in a sparking plug at an unexpected timing. PA0 (2) Where a circuit for monitoring the drain voltage in order to monitor the operating status of the ignition system, the oscillation of the drain voltage immediately after the current limitation is started is harmful. PA0 (3) The oscillation of the drain voltage waveform may lead to the oscillation during the entire period of the current limitation.
On the other hand, the reason why the bipolar Darlington transistor gives a very slight amount of the collector voltage oscillation immediately after start of the current limitation unlike the MOS gate structure transistor is attributable to that it has an entirely different output characteristic (which can be exhibited as a collector voltage in abscissa and a collector current in ordinate) from that of the MOS gate structure. FIG. 4 shows the output characteristic of the bipolar Darlington transistor actually used in a vehicle engine ignition circuit. The operating waveform when using this transistor is shown in FIG. 2. Further, FIG. 5 is an output characteristic chart of the MOS gate structure transistor (MOSFET in the present case) providing the waveform of FIG. 3. It is of course that IGBT provides the output characteristic similar to MOSFET. FIG. 4 is greatly different from FIG. 5 in the changing degree of the collector current for an increase in the collector voltage of 2 V or more. It can be seen from FIG. 4 that the bipolar Darlington transistor exhibits a greater change in the collector current.
The mechanism of less oscillation of the collector voltage in the bipolar Darlington transistor can be explained as follows. As described above in connection with the prior art, as the voltage across resistor 106 increases in proportion to an increase in the collector current (which is also an ignition coil current), a base current flows into the transistor 104 so that its collector-emitter path becomes conductive.
Then, in response to conduction of the transistor 104, a part of the current having been flowing as the base current to the Darlington transistor 103 through the resistors 108 and 107 is shunted as a collector current to the transistor 104. When the voltage across the resistor 106 is further increased, it operates to reduce the base current to the transistor 104 and to increase the base current to the transistor 103. Eventually, the voltage across the resistor 106 substantially depends on the base-emitter voltage characteristic of the transistor 104 so that the collector current of the transistor 103 is maintained constant. On the other hand, there is necessarily a time lag from when the base current starts to flow into the transistor 104 to when the collector current of the transistor 103 becomes constant. Thus, the base current to the transistor 103 gradually decreases from the value defined by the voltage of the battery 101 and resistors 108, 107 during the above time lag.
The mild increase in the collector voltage before the current limitation operation starts in the operation waveform in FIG. 2 should be attributable to the base current gradually reducing and the output characteristic of the bipolar Darlington transistor shown in FIG. 4. This mild increase in the collector voltage makes a change in the collector current immediately before the current limitation operation starts. The mild changes in the collector voltage and the collector current contribute to suppression of oscillation in the collector voltage.
Further, where there is a great change in the collector current at the collector voltage of about 2 V or more as shown in FIG. 4, even if the above time lag is zero so that the base current to the transistor 103 changes stepwise from the value defined by the voltage of the battery 101 and the resistors 108 and 107, the oscillation in the collector voltage immediately after the current limitation is anticipated to be very small for the following reason. The rise (oscillation) in the collector voltage immediately after the current limitation starts does not occur as long as the secular change in the ignition coil current does not shift from the increase to the decrease. Namely, when the secular change in the collector current shifts from the increase to the decrease and the collector voltage is going to rise at a certain base current, the collector current is relatively greatly increased in the transistor having an output as shown in FIG. 4. This increases the collector current which is decreasing. In other words, the transistor itself has a negative feedback function that the collector voltage increases for a decrease in the collector current. The negative feedback function makes difficult the shift of the ignition coil current from the increase to the decrease, thereby suppressing oscillation of the collector voltage.
On the other hand, as shown in FIG. 5, since a change in the collector current in the MOS gate transistor is very small at the collector voltage of about 2 V or higher, the increase in the collector current due to the increase in the collector voltage is very small. Therefore, the negative feedback function is very weak so that oscillation of the collector voltage is not suppressed.